Amorphous precipitated siliceous pigments and methods for their production

ABSTRACT

A new method for producing precipitated silicas having a unique combination of physical and chemical properties is disclosed. The silicas are produced by acidulating a solution of an alkali metal silicate having a specific SiO 2  /Na 2  O mol ratio with an acid until precipitation just beings. At this point, the reaction mass is aged for a period of time and thereafter the acid addition is continued until the precipitated product is obtained. Products produced in accordance with the invention exhibit lower wet cake moisture and are characterized by their low structure, low oil absorption, high abrasiveness and high pack density, and as such are distinctly different from silicas used as reinforcing fillers in rubber. In a particularly advantageous embodiment, an adduct material, such as aluminum, is added to control the refractive index of the precipitated pigment. Products produced in this manner have particular utility for use as abrasion and gelling agents in clear toothpaste compositions.

RELATION TO COPENDING APPLICATIONS

The present application is a continuation of Ser. No. 519,720, filedOct. 31, 1974, now U.S. Pat. No. 3,988,162, which is in turn acontinuation-in-part of U.S. Ser. No. 286,655, filed Sept. 6, 1972, nowU.S. Pat. No. 3,893,840.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the production of precipitated silicasand, more particularly, to a novel process for producing syntheticprecipitated silicas and silicates having new and improved physical andchemical properties.

2. Description of the Prior Art

As known in the art, finely divided amorphous precipitated silicic acidpigments and certain zeolitic type alumino silicates may be prepared bythe acidulation of an aqueous silicate solution with an acid or a saltof the acid, such as aluminum sulfate. Such products are commerciallyavailable being sold, e.g., under the trademarks "Zeo"; "Zeolex"; and"Arogen" by the J. M. Huber Corporation. Specific examples of theseproducts as well as methods for their preparation are disclosed in U.S.Pat. Nos. 2,739,073; 2,843,346; and 3,582,379.

Prior to the present invention, known and commercially available silicaswere characterized by the following properties: high structure, high wetcake moisture content, high oil absorption, low valley abrasion, highsurface area, and low pack density. In this regard, and due in part tothe properties such as high oil absorption, high surface area, etc., thepigments have been widely and successfully used as reinforcing pigmentsin rubber.

However, the high wet cake moisture content is disadvantageous in thatthe drying and filtration rates are increased, thus increasing theoverall cost in the production of the final product. For example, inknown and conventional methods of producing silicic acid pigments orsilicas, the wet cake moisture content of the product (followingfiltration of the precipitated reaction mass) is approximately 82%. Thismeans that there can be recovered only 18 parts of dry pigment from 100parts of wet cake.

In this regard and as noted above, there are a number of knowntechniques for preparing silica pigments which involve acidulating anaqueous silicate solution. Thus in U.S. Pat. No. 2,940,830 which issuedJune 14, 1960 to F. S. Thornhill, there is described a process forpreparing finely divided silicas which are suitable as reinforcingagents in rubber compositions. Thornhill more specifically describes aprocess of preparing a silica material which is characterized by havingan average ultimate particle size of 0.015 to 0.04 micron and a surfacearea of 25 to 200 square meters per gram by the controlled rate ofaddition of acid to an alkali metal silicate wherein the resultantslurry is constantly maintained at a pH above 7 in order to achieve theaforementioned end product characteristics. The Thornhill patent isspecificaly directed to the production of a product suitable as areinforcing agent in rubber compositions.

In U.S. Pat. No. 3,235,331 which issued Feb. 15, 1966 to Nauroth et al.,there is described a process for producing a precipitated silica whichis also stated to be useful as a reinforcing agent for rubber. Morespecifically, this patent discloses a process wherein an aqueous alkalimetal silicate solution and acid are simultaneously added to a reactionvessel. In the Nauroth et al. patent, it is pointed out that thissimultaneous addition is continued until the viscosity of the pool risesthrough a maximum and then falls to a substantially lower value. Theamount of the acidification agent and the alkali metal silicate areproportioned so as to maintain the pH of the resulting slurrysubstantially constant throughout the major portion of the reaction andin the range of about 10 to 12. The process is generally conducted at atemperature of 80° to 90° C. and the end product, after drying, resultsin a silica which may have a surface area of 260 square meters per gram.The patentees point out that the product is satisfactory as areinforcing agent for rubber.

In U.S. Pat. No. 3,445,189 issued May 20, 1969 to Matt et al., there isdescribed a process for producing finely divided silicic acid bysimultaneously adding solutions of an alkali silicate and a strongmineral acid to water at a temperature between 70° C. and 90° C. whilemaintaining the reaction pH between 7 and 9. The patentees point outthat the product obtained by the aforementiond process is a finelydivided nongelatinous silicic acid which is useful as a filler fornatural and synthetic rubber and other elastomers. It is also disclosedin this patent that for a silica to be useful as a filler for naturaland synthetic rubber and other elastomers, its surface area and oilabsorption are of vital importance. This patent further discloses thatextensive investigations have further indicated that if a finely dividedsilicic acid is to have good reinforcing properties for rubber, it musthave a surface area of 100 to 250 m² /g and an oil absorption of morethan 2 cc/g or 200 cc/100 g. See column 2, lines 18 through 22.

In U.S. Pat. No. 3,730,749 which issued May 1, 1973 to James E. Morgan,there is disclosed a process for preparing silica for use in reinforcingcompositions. It is pointed out in Morgan that the viscosity increasewhich occurs during the acidification or neutralization of aqueousalkali metal silicate is substantially minimized by adding a controlledamount of an alkali metal silicate. In Examples I, II, and III of thispatent, it is also noted that the silica filter cakes had solid contentsof 18.5; 24.9; and 25.1 percent, respectively. This means that thepercent wet cake moisture of the silicas disclosed in Examples I, II,and III is one hundred minus the percent solid content in the filtercake. In other words, the percent wet cake moisture (% WCM) of silicasmentioned in Examples I, II, and III is 81.5; 75.1; and 74.9,respectively. The surface area, the average ultimate particle sizes, andrubber data of silicas produced by the teachings of Examples II and IIIare listed in Table 3 which also sets forth that rubber compositionsincorporating the silicas of Examples II and III have desirable rubberproperties. It is further substantiated by this patent that rubberproperties of silicas are related to the wet cake moisture of the silicapigment. A silica of high percent wet cake moisture and suitableparticle size and surface area has better rubber properties than thecorresponding material of lower wet cake moisture. Thus the silicasdisclosed in Morgan have a higher structure index, and therefore thesilicas are useful rubber reinforcing fillers.

From the above it will be seen that the structure index of a silica isrelated to the rubber properties -- a silica of higher structure indexwill have better rubber properties than a silica of lower structureindex. At this point, the various types of synthetic silicas, as well as"structure" and "structure index" should therefore be discussed.

In this regard, and as known in the art, commercially availablesynthetic silicas are derived either by a liquid phase or a vaporprocess. Silicas obtained by the vapor process are called fumed orpyrogenic silicas. Products obtained by the liquid process arecategorized as silica gels and precipitated silicas. Thus, there arethree distinct types of synthetic silicas on the market:

1. Pyrogenic Silicas

Pyrogenic or fumed silicas are prepared by reacting silicontetrachloride vapor with oxygen and hydrogen gas at high temperatures.These products have high external surface areas and differ from othersilicas (e.g., gels, precipitated silicas) prepared from the liquidphase process. Cabot and DeGussa are two suppliers of pyrogenic silicas.

2. Silica Gels

Silica gels are of two types -- hydrogels and aerogels. Hydrogels areprepared by reacting a soluble silicate such as sodium silicate withstrong sulfuric acid. The gel is washed salt-free, dried, steammicronized, and then classified. Aerogels are prepared from crudehydrogels by displacing its water content with an alcohol. The alcoholis then recovered by heating the gel in an autoclave.

Aerogels are lighter and fluffier than hydrogels because the shrinkageof the gel structure is avoided during the drying process. Gels havevery large surface areas, generally in the range of 300-1,000 m² /g andhigh porosities. Silica gels are offered, e.g., by W. R. Grace andCompany under the trademark "Syloid"; by Monsanto under the trademark"Santocel"; and Glidden under the trademark "Silcron."

3. Precipitated Silicas

Precipitated silicas are produced by the de-stabilization andprecipitation of silica from soluble silicate by the addition of amineral acid and/or acidic gases. The reactants thus include an alkalimetal silicate and a mineral acid, such as sulfuric acid or anacidulating agent such as CO₂.

When the acidification agent is added to the alkali metal silicate, at acertain point during the process, the silica starts precipitating. Theaddition of the acidification agent is continued until the M₂ O of thealkali metal silicate (M being the alkali metal) of the ultimate silicais less than about 1% by weight. Thus, as a general rule, theacidification agent is added to the alkali metal silicate to neutralizethe alkali portion bound to the silicate anion. The reaction slurry isfiltered and washed free of reaction by-product, which is the alkalimetal salt of the acidification agent. The filter cake is dried andmilled to obtain a silica of desired degree of fineness.

Prior to the drying step, the silica filter cake generally results in afilter cake which contains a surprisingly high amount of water. Forexample, a silica which is useful as a filler for reinforcement ofrubber and elastomers generally contains 80% to 85% water in its cake.For example, see Example No. I, U.S. Pat. No. 3,730,749 where the % wetcake moisture is 81.5. The percent water present in the filter cake isknown as percent wet cake moisture or generally abbreviated as "% WCM."One hundred minus the % WCM gives the solid content of the filter cake,i.e., the amount of silica which can be recovered in the solid form upondrying the filter cake. The percent solid content of the filter cake istermed percent filter cake solids and generally abbreviated as "% FCS."Thus, % WCM and % FCS are related by the equation:

    % WCM = 100 - % FCS

or

    % FCS = 100 - % WCM

if we know the value of % WCM, we can calculate % FCS or vice versa.

Thus, a silica filter cake having 85% WCM will have 100 minus 85 of 15%FCS. This means that 15 pounds of silica can be recovered from such afilter cake by evaporating or drying 85 pounds of water from hundredpounds of filter cake. The total weight of filter cake consists of waterand solid silica. In the example where the % WCM is 85, one can recoveronly 15 pounds of solid silica as can be seen below:

    100 pound filter cake = 85 pounds water + 15 pounds dry silica = 85% WCM + 15% FCS

thus, there are 85 pounds of water associated with 15 pounds of solidsilica content of 85/15 × 100 = 567 pounds of water per 100 pounds ofsolid silica.

The water associated with the silica content of filter cake isstructural water. This water is present whereby it occupies theavailable space between the silica aggregates and also the space insidethe silica aggregates or micelles, micelles being defined by TheAmerican Heritage Dictionary of the English Language, College Edition,Copyright 1969, 1970, 1971, 1973, 1975, 1976; Houghton Mifflin Company,Page 828, as "a submicroscopic aggregation of molecules such as adroplet in a colloidal system; an organic particle of colloidal sizefound in coal; a coherent strand or structure in natural or syntheticfibers; a submicroscopic structural unit of protoplasm". As used herein,the term "structure" is defined as the ability of a silica to hold waterin its wet cake. When silicas, such as the aforementioned known priorart products, hold a high percentage of water, i.e., from about 70 to85%, they are known as high structure silicas. Materials holding lessthan 70% or from about 50 to 75% are referred to as low structuresilicas. This total structural water content is a very importantproperty of silica and is directly related to the functional and end useproperties of silica. The amount of total structural water associatedwith 100 pounds of solid silica content of the filter cake is defined as"structure index" and abbreviated as S. I.

Mathematically, structural index (S. I.) of silica can be calculated ifeither the % wet cake moisture (WCM) or the % filter cake solid (FCS)values of said silica are known: ##EQU1## Structure index of silicas inwet cake moisture range of 80-85% is listed in Table I.

                  Table I                                                         ______________________________________                                        Structure Index of Silicas With                                               % WCM of 85 - 80                                                              % WCM       100 - % WCM     S. I.                                             ______________________________________                                        85          15              567                                               84          16              525                                               83          17              488                                               82          18              455                                               81          19              426                                               80          20              400                                               ______________________________________                                    

in addition to the above-discussed properties of known silicas, i.e.,high wet cake moisture, structure index and high oil absorption, the lowabrasiveness and high refractive index of known silica and silicatepigments renders them unsuitable for many uses. For example, it is wellknown that conventional synthetic precipitated silicas are unsuitable aspolishing and abrasive agents in toothpaste compositions. See GermanPat. No. 974,958; French Pat. No. 1,130,627; British Pat. No. 995,351;Swiss Pat. No. 280,671; and U.S. Pat. No. 3,250,680. In this regard, itis disclosed in U.S. Pat. No. 3,538,230 that known amorphous silicassuch as precipitated silicas, pyrogenic silicas and aerogels areunsuitable for dentrifrice use because they show substantially nocleaning ability on human teeth because of their initial small particlesize and because of the ease in which they break down into smallparticle sizes which result in poor cleaning ability.

Further, and in more detail, conventional silicas and amorphousprecipitated alumino silicates, such as "Zeolex" and "Arogen," cannot beused for a clear gel toothpaste because of their high refractive index(1.55) and because they lack the needed abrasive and polishingcharacteristics when added to the toothpaste base composition. Clear geltoothpaste contains a high percentage of abrasive and polishing agent inthe toothpaste formula. The major function of the abrasive and polishingagent is to remove stains, food debris, and bacterial plaque from thehuman tooth surface. Ideally the polishing agent should provide amaximum cleaning action at acceptable abrasion levels and must becompatible at high loadings of 15% up to 50% with other toothpasteformula ingredients. Thus known silicas and alumino silicates areunsuitable for clear gel toothpastes, (such as the product sold underthe Trademark "Close-Up" by Lever Brothers) because they cannot be addedat high loadings of 15% and above in a typical toothpaste composition.Because of their high oil absorption, high sorption characteristics andhigh refractive index (1.55) known precipitated pigments thicken up thedentifrice composition and impart undesirable opacity to the base pasteresulting in an unacceptable product. In summary of the above,precipitated silicas and silicates cannot be used in conventional andclear gel dentifrice compositions because such products result inunacceptable toothpaste consistencies and do not possess the acceptableabrasive and polishing characteristics needed for use in dentifricecompositions.

SUMMARY OF THE INVENTION

In summary, the present invention relates to the production of syntheticprecipitated silicas having new and improved physical and chemicalcharacteristics. More particularly, the invention is directed to a newand unique process for producing precipitated silicas and silicateshaving low structure, low wet cake moisture content, high abrasion, lowsurface area, low oil absorption and high pack densities. Because ofsuch properties the improved pigments can be advantageously andeffectively used as an abrasive and polishing agent in dentifricecompositions. In addition, the new products can be advantageouslyemployed in further applications such as in the preparation of molecularsieves, as flattening and texturizing agents, as carriers, viscositycontrol agents, etc.

Stated broadly, the method of the invention embodies the concept and isbased on the discovery that if the addition of the acid to the silicatesolution is interrupted at the first appearance of the opalescence point(i.e., that point at which precipitation first begins) the resultingpigments possess the aforementioned unique combination of properties. Inaccordance with the invention, the low structure pigments of theinvention are prepared by acidulating the aqueous alkali metal silicatesolution (which as to be discussed, has a specific SiO₂ /Na₂ O molratio) until precipitation just begins. At this point the reaction massis aged for a suitable length of time generally on the order of fromabout 15 to 20 minutes. After the aging period, the introduction of theacid is continued until the precipitated product is obtained. Inaccordance with a further method embodiment it has been found that theproperties of the pigment can be closely and accurately controlled ifthe acid and a portion of the alkali metal silicate are addedsimultaneously so that the reaction is carried out at an essentiallyconstant pH. In accordance with a third and particularly advantageousmethod embodiment, the acidulating agent, such as sulfuric acid, ispre-mixed with an adduct such as aluminum (which is preferrably added asa water soluble salt thereof, as e.g., aluminum sulfate). In this regardit has been found that the addition of the adduct, coupled with theaging of the reaction mass, substantially controls the refractive indexof the resulting pigment while at the same time not effecting thesignificant increase in the abrasiveness.

As briefly noted above, the precipitated pigments produced in accordancewith the unique method of the invention results in materials of lowerprocessing costs, better packaging characteristics and a unique balanceof physical chemical properties as compared to conventionallyprecipitated silicas.

It is accordingly a general object of the present invention to provide anovel process for producing precipitated silicas having improvedphysical and chemical properties.

Another and more particular object is to provide a unique process forproducing synthetic amorphous precipitated siliceous pigments which haveparticular utility for use as abrasive and polishing agents indentifrice compositions.

Yet another object is to provide a highly efficient and improved processfor producing silicic acid pigments which exhibit lower wet cakemoisture or higher percent solids and which have high valley abrasionand low oil absorption characteristics.

A further object is to provide a new process for producing precipitatedamorphous silicas which have a unique balance of physical and chemicalproperties as compared to conventionally known precipitated pigments,said process further resulting in lower processing cost.

A still further object is to provide a process for producing lowstructure, low wet cake moisture, low surface area, low oil absorption,high valley abrasion, and high pack density precipitated silicic acidpigments.

BRIEF DESCRIPTION OF THE DRAWINGS

The manner in which the foregoing and other objects are achieved inaccordance with the present invention will be better understood in viewof the following detailed description and accompanying drawings, whichform a part of the specification and wherein:

FIG. 1 is a graph showing the effect of the silicate mol ratio on thesurface area of pigments produced in accordance with the invention andthose produced by prior known techniques.

FIG. 2 is a graph showing the effect of the silicate mol ratio on thewet cake moisture content for pigments produced in accordance with theinvention and those produced by prior techniques.

FIG. 3 is a graph illustrating the abrasivity verses the structure indexof silica.

DESCRIPTION OF PREFERRED EMBODIMENTS

As briefly discussed above, when an acid is added to a solution of analkali metal silicate, the resulting reaction medium remains clear untilsuch point that a slight turbidity (called the opalescence point)appears. As the acid addition is continued, the silicic acid or silicapigments start precipitating until all of the silicate solution isprecipitated. In conventionally known processes, the pH of the reactionmass is then adjusted to a range of from about 5.5 to 6.5 and the massis filtered, washed, and dried.

In accordance with the present invention, it has been discovered that ifthe acid addition is interrupted for a suitable period of time at thefirst appearance of opalescence, the resulting pigment possesses aunique combination of physical and chemical properties as well asimproved processing advantages. More specifically and again as brieflynoted above, the new silicic acid pigments produced in accordance withthe invention exhibit lower wet cake moisture (or higher percent solids)thereby permitting increased drying and filtration rates. Further thenew pigments have been found to have low structure, relatively lowsurface areas, low oil absorption, high pack densities and high valleyabrasion. Because of these properties, the new pigments are particularlysuitable and adapted for use as an abrasive and polishing agent intoothpaste compositions. Other properties include controlled particlesize, better dispersion and improved wetting and viscositycharacteristics.

At this point it may be noted that alkali metal silicate solutions usedor proposed for use in such prior art can be expressed by the formula,M₂ O/(SiO₂)_(X), wherein M is an alkali metal, such as sodium orpotassium, and X is the number of moles of SiO₂ bound to M₂ O. Where M =sodium, and X = 3.3, the resulting sodium silicate solution, i.e., Na₂O/(SiO₂)₃.3 is commonly known in the literature as water glass. When thevalue of X = 1, the product is called sodium metasilicate and has thecomposition Na₂ O.SiO₂. Similarly, when X = 2, the product is calledsodium disilicate and has the composition: Na₂ O/(SiO₂)₂. The mostextensive work reported in the literature and prior art with regard tothe production of silica products as per the acidulation of a silicatesolution, has been with silicates of the composition Na₂ O/(SiO₂)₃.3,the latter as aforesaid commonly being referred to as water glass. Theuse of water glass produces a pigment having certain specificproperties, namely high structure, high oil absorption, high surfaceareas, and low valley abrasions. See U.S. Pat. Nos. 2,940,830;3,235,331; 3,445,189; and 3,730,749. See also Canadian Pat. Nos. 751,308(Burke) and 619,718 (Hall).

While prior work reported in the literature has disclosed the use ofsilicates of the composition Na₂ O/(SiO₂)_(X) wherein X = 1 to 4, in sodisclosing the use of such silicates (wherein X = 1 to 4) they have beenreferred to as equivalents.

However, in research efforts precedent to the present invention, aseries of tests were initially conducted wherein silicates of varyingSiO₂ /Na₂ O mol ratios were employed. The results of several of thesetests are set forth hereinbelow.

Experiment I

    ______________________________________                                        Raw Materials                                                                 ______________________________________                                        Silicate Solution                                                                              Na.sub.2 O/(SiO.sub.2).sub.3.3, Volume 10 Gal.               Sodium Silicate Concentration                                                                  13.3%                                                        Sulfuric acid concentration                                                                    11.4%                                                        Reaction Temperature                                                                           175°0 F                                               ______________________________________                                    

When the above acid was added to the silicate solution at 453 ml/min, asilica pigment having the following properties was obtained:

    ______________________________________                                        Product A                                                                     ______________________________________                                        Wet cake moisture  =        86%                                               Surface area       =        322 m.sup.2 /g                                    Oil absorption     =        200                                               ______________________________________                                    

Experiment II

Experiment I was repeated except that the silicate used had thecomposition Na₂ O/(SiO₂)₂.5. The resulting silica pigment had thefollowing properties:

    ______________________________________                                        Product B                                                                     ______________________________________                                        Wet cake moisture  =        82%                                               Surface area       =        130 m.sup.2 /g                                    Oil absorption     =        200                                               ______________________________________                                    

The results of the above tests were unexpected.

it was next discovered that further unique properties could be obtainedif the addition of the acid was interrupted for a period of time at thatpoint at which precipitation of the pigment just begins. The results ofthese tests are set forth in the Examples hereinbefore. In effect thesetests established that silicates of the composition M₂ O/SiO₂)_(X) wherein X = 1 to 4 are not equivalents and that when a silicate having a SiO₂/M₂ O mol ratio in the range of from about 2.0 to 2.7 is employed, theresulting pigment had certain unique properties characterized generallyas a low structure, high abrasion, low surface area, low oil absorptionand high pour densities. Specifically, products produced in this mannerhave a wet cake moisture content of less than 70%; a surface area ofless than 100 m² /g; an oil absorption of less than 125 cc/100 gn; packdensities of greater than 12 pounds/cu. ft. and a valley abrasion ofgreater than 5.0 mg wire loss.

Turning now to more specific details, in the practice of the invention asolution of the alkali metal silicate is first charged to a reactor andthe solution is heated to a temperature in the range of from about 100°F. to 200° F., preferrably on the order of from about 150° F. to 175° F.Particularly advantageous and superior results are obtained if theconcentration of the silicate solution is on the order of from about 1.0to about 2.5 pounds/gallon. The acidulating agent or acid, e.g.,sulfuric acid, is next charged to the reactor until the slight turbidity(i.e., the opalescent point) first occurs. At this time the acidaddition is stopped and the reaction is aged for a period of time on theorder of from about 10 minutes to 1 hour. As to be discussed in moredetail hereinafter, while the point or time at which the acidulation isdiscontinued is critical, it has been found that the aging period isgenerally dictated by process economics. For example, although thereaction mass must be aged for at least 10 minutes to obtain theaforementioned unique combination of properties, it has been found thataging for a period of longer than 2 hours does not, in fact, produce anyparticular advantage. Therefore from an economic standpoint, the agingperiod is preferably on the order of from about 10 to 15 minutes.

In accordance with one method embodiment of the invention, it has beenfound that a more homogeneous product can be obtained when from aboutone-half to two-thirds of the total silicate is initially charged to thereactor, and the remaining silicate is added simultaneously with theacid in a manner such that the reaction is carried out at asubstantially constant pH, preferrably on the order of from about 8.5 to10.5. After the pigment has been precipitated, the pH of the resultingslurry is reduced to from about 5.5 to 7.0 by the addition of an excessof the acid.

In a further method embodiment, it has been found that the refractiveindex of the precipitated pigment can be controlled by the addition ofan adduct element (such as aluminum, magnesium, and the like) to providean abrasive or polishing agent for a clear translucent or transparenttoothpaste composition. Thus in this embodiment, the acid is premixedwith a solution of the adduct material, i.e., aluminum (preferrably inthe form of a water soluble salt such as aluminum sulfate, etc.) and theacid-metal salt mixture is then used for acidulating the aqueous alkalimetal silicate until the precipitation just begins. At this point, andsimilarly as in the other embodiments, the acid and metal salt additionis interruped and the reaction slurry is aged. After the aging, theaddition of the acid-metal salt mixture is continued until theprecipitation of the product is complete.

As will be seen from the above, the starting materials or reactantsemployed in the present invention include certain alkali metalsilicates, an acid and a water soluble metal salt. As used herein theterm alkali metal silicates include all the common forms of alkali metalsilicates. Water soluble potassium silicates and sodium silicates areparticularly advantageous. Because of their relatively low cost, sodiumsilicates are preferred. In this regard commercially available sodiumsilicate solutions are more or less polymerized depending on theirsilica to sodium oxide (SiO₂ /Na₂ O) ratios. For example, sodiummetasilicate solution (mole ratio unity) is known to be predominatelymonomeric in character while water glass (mole ratio 3.3) is bothmonomeric and polymeric in character. As the silica to sodium oxide moleratio of sodium silicate increases, so does the polymer to monomer ratioof its silicate anions. As discussed above, particularly advantageous,superior, and unexpected results are obtained in the SiO₂ /Na₂ O ratiois in the range of from about 2.0 to 2.7. In other words, the latterratio is critical and forms a part of the discovery upon which theinvention is based.

While the acidulating agent or acid is preferably a strong mineral acid,such as sulfuric acid, nitric acid, and hydrochloric acid, it should beunderstood that other acids, including organic acids, as for example,acetic acid, formic, or carbonic acid can be employed. The adductmaterial, employed to control the refractive index of the precipitatedproduct may comprise metals such as aluminum, magnesium, zinc, andcalcium. However, the adduct is preferrably employed in the form of awater soluble salt of the metal which should be compatible with the acidused for precipitation. For example, aluminum salts useful in the methodof the invention are the water soluble salts of aluminum and strongacids such as aluminum sulfate, aluminum chloride, aluminum nitrate, andammonium alum. The amount of the adduct or metal employed may varydepending upon the particular refractive index required. As shown in thefollowing examples, an excess of the adduct (e.g., Al₂ (SO₄)₃) willincrease the refractive index to a level above that required for cleardentifrice compositions (i.e., 1.475). However, refractive indices above1.475 may be particularly suitable for many applications and the gist ofthis method embodiment lies in the discovery that the use of the adductserves to control this property. The acidulating agent or acid ispreferrably added as a dilute solution thereof. However, preferred andsuperior results are obtained if the acidic solution is from about 10 to25% by weight acid based on the total weight of the solution.

The invention will be further illustrated by the following Examples.

EXAMPLE 1

Thirty gallons of a 1.24#/gal. sodium silicate solution (SiO₂ to Na₂ Omolar ratio of 2.5) was added to the stirred reactor and the silicatesolution was heated to 185 degrees F. Sulfuric acid of 11.2%concentration was added to the reactor at the rate of 0.81 gallons perminute till a pH of 10.0 plus or minus 0.1 was reached. At this pH, theprecipitation of silica micelles just started. The acid was shut off andthe reaction medium was aged for ten minutes. After the aging period,both acid and silicate were added simultaneously at the rate of 0.84 and1.0 gallons per minute. The silicate was turned off after thirtyminutes, the acid addition was continued and the batch was finished offat pH 5.8, filtered, washed, dried, and milled. The results of this andfurther examples are shown and summarized hereinbelow.

EXAMPLE 2

In this experiment, 35 gallons of 1.24#/gal. silicate of SiO₂ to Na₂ Omolar ratio of 2.5 were added to the stirred reactor and the silicatesolution was heated to 175 degrees F. Sulfuric acid of 11.4%concentration was added to the reactor at the rate of 0.84 gallonsperminute till a pH of 10.1 plus or minus 0.1 was reached. At this pH,the precipitation of silica micelles just started. The acid was shut offand the reaction medium was aged for fifteen minutes. The aging step isimportant to obtain homogeneous product and for the silica micelles toreach an aquilibrium condition. After the aging period, both acid andsilicate were added simultaneously at the rate of 0.84 and 1.4 gallonsper minute. Silicate was turned off after twenty-five minutes; acidaddition was continued and the batch was finished off at pH 5.5,filtered, washed, dried, and milled. In further tests it was found thata range of products can be made by introducing more than one aging stepduring the process and by maintaining the precipitation pH substantiallyconstant and within the range of pH 7-10.

EXAMPLE 3

In this experiment, ten gallons of 1.24#/gal. sodium silicate of SiO₂ toNa₂ O molar ratio of 2.5 were added to a stirred reactor and thesolution heated to 175 degrees F. Sulfuric acid of 11.4% concentrationwas added to the reactor at the rate of 0.12 gallons perminute until thesilica just started precipitating. At that time the reaction pH was10.1. The acid addition was stopped and the reaction medium was aged forfifteen minutes. After the aging period, the acid addition was resumedagain until the reaction pH of 9.0 was achieved. At this point the acidwas stopped again for 15 minutes and the reaction medium was aged. Afteraging period was over, silicate was only added to the reactor at therate of 0.1 gallons per minute until pH of 10.1 was reached. Acidaddition was resumed again and the batch was finished off at pH 5.7. Theidea of aging at pH 9.0 was to have the silica particles grow to abigger size and to obtain a final product with lower structure, lowersurface area and higher pack density than the conventional silica.

EXAMPLE 4

In this example, five gallons of 1.24#/gal. sodium silicate of SiO₂ toNa₂ O molar ratio of 2.5 were added to the stirred reactor and thesilicate was heated to 175 degrees F. Acid of 10.5% concentration wasadded to the silicate till a turbidity or faint precipitation appearedin the reactor. Acid was stopped at this point and the reaction mediumwas aged for twenty minutes. After the aging period, five gallons ofsilicate and 11.4% acid were added simultaneously at the rate of 0.20 to0.12 gallons per minute and the batch was finished off at pH 5.5,filtered, washed, dried, and milled.

EXAMPLE 5

The procedure of Example 4 was repeated except that six gallons ofsilicate were added to the reactor and the remaining four gallons wereadded after the aging period along with the acid.

EXAMPLE 6

The procedure of Example 4 was repeated except that seven gallons ofsilicate were added to the reactor and the remaining three gallons wereadded simultaneously with acid after aging the reaction medium. Thebatch was processed similar to Example 4.

EXAMPLE 7

The procedure of Example 2 was repeated except that after the agingperiod, the precipitation pH was controlled at pH 9.9 plus or minus 0.1.The batch was finished and processed similar to Example 2.

EXAMPLE 8

The procedure of Example 4 was repeated except that the finishing pH wasbrought down to 3.2. Lower finishing pH results in product of highersurface area.

EXAMPLE 9

In this example, 71/2 gallons of 1.24#/gal. sodium silicate were addedto the stirred reactor and heated to 175 degrees F. Acid of 11.4%concentration was added till the precipitation of silica particles justbegan. Acid was then stopped and the reaction medium was aged forfifteen minutes. After the aging period, 71/2 gallons of silicate and11.4% acid were added simultaneously at the rate of 0.3 GPM and 0.18 GPM(gallons per minute) respectively, and the batch was finished off at pH5.6, and the batch was processed similar to Example 4 above.

EXAMPLE 10

A control batch of conventional precipitated silica was prepared byneutralizing 1.24#/gal. sodium silicate with 11.4% acid till the finalpH of 5.5 was obtained. In the control batch no aging step was involvedso that the properties of material produced via the new process could becompared with the control batch.

Data on precipitated silicas obtained in Examples 1 thru 10 aresummarized below in Table II.

                                      TABLE II                                    __________________________________________________________________________                 %     Surface                                                                            Oil   Density                                                                             Valley                                                 Wet Cake                                                                            Area Absorption                                                                          Pour, Pack                                                                          Abrasion                                  Example                                                                            Description                                                                           Moisture                                                                            (m.sup.2 /g)                                                                       cc/100g                                                                             (=/cu. ft)                                                                          mg loss                                   __________________________________________________________________________    1    Silica via aging                                                                      65    66   85    11.2 18.7                                                                           15.0                                      2    Silica via aging                                                                      55    38   81    14.2 27.7                                                                           17.3                                      3    Silica via aging                                                                      70    66   --     9.9 19.5                                                                           6.7                                       4    Silica via aging                                                                      53    29   66    18.4 33.7                                                                           83.5                                      5    Silica via aging                                                                      59    116  108   14.2 26.0                                                                           15.3                                      6    Silica via aging                                                                      59    46   108   13.9 26.0                                                                           16.3                                      7    Silica via aging                                                                      62    80   83    12.5 22.2                                                                           18.8                                      8    Silica via aging                                                                      60    242  95    12.7 23.9                                                                           28.4                                      9    Silica via aging                                                                      56    38   81    17.8 29.7                                                                           18.6                                      10   Control 82    150  211    6.3 10.7                                                                           2.5                                            (no aging)                                                               __________________________________________________________________________

From the above data, it is clear that the new process of the inventionresults in silicas of lower wet cake moisture, lower structure, loweroil absorption, lower surface area, higher pack density and highervalley abrasion than conventional precipitated silicas.

The new process leads to silicas of lower processing costs than regularprecipitated silicas. For example, the average wet cake moisture ofsilicas via the new process is only 60% as opposed to 82% for regularsilica (see control). This means we can recover 40 parts of dry silicafrom 100 parts of wet cake if the silica is produced via the newprocess. Regular processes result in recovery of about 18 parts of drysilica per 100 parts of wet cake. Thus, via the new process we canrecover 22 parts more of dry silica or an increase of (22/18) × 100 or122%. The new process results in silicas of better drying and filtrationrates and hence significantly lower processing costs than theprecipitated silicas produced by the conventional process.

EXAMPLE 11

In a series of tests the general procedures of Examples 1-10 wererepeated except that the precipitating pH and the period of aging werevaried. The pH was varied in the range of from about 5.5 to 11.0. Theaging period was varied from about 5 minutes to 1 hour. The results ofthese tests were substantially the same as that of Examples 1-10 exceptthat it was found that products of predetermined properties andcharacteristics (i.e., a specific structure or wet cake moisturecontent) could be obtained by varying the above process conditionswithin the specified ranges. These tests also established that if thefinal batch pH is reduced to below about 5.0, an increase in the surfacearea was obtained. Thus, if it is desired that the final product havelow surface area, then the pH of the final batch should be maintainedabove 5.0. Also these tests established that aging for periods of lessthan about 8-10 minutes was generally ineffective to produce the lowstructures, etc., products of the invention.

EXAMPLE 12

The general procedure of Examples 1-11 were repeated except that nitricacid, hydrochloric acid, acetic acid, and formic acid were substitutedfor the sulfuric acid. The results were substantially the same as inExamples 1-11.

EXAMPLE 13

In a series of tests the general procedures of Examples 1-12 wererepeated except that aqueous sodium silicates having mol ratios (SiO₂/Na₂ O) of 2.0; 2.2; 2.4; and 2.7 respectively, were substituted for the2.5 silicate of Examples 1-12. The results established that remarkableresults were obtained when these mol ratios were used. The wet cakemoisture content for these tests were 61.2; 60.5; 60.1; and 62,respectively. The surface areas (m² /g) were 70.9; 65.0; 55.0; and 75.0respectively. It was also established that these ratios were required toprepare a product having a wet cake moisture content of less than 70%;an oil absorption of less than 125 cc/gm.; pack densities of greaterthan 12 pounds/ft³ ; valley abrasions greater than 5.0 mg wire loss anda surface area of less than 100 m² /gm (no adduct). The adduct increasedthe surface area up to about 300 m² /gm. In a series of further teststhe wet cake moisture content and the surface area were compared forsilicic acid pigments produced by prior known techniques (non-aged) andproducts produced in accordance with the present invention. The resultsare shown in FIGS. 1 and 2.

The following Examples serve to illustrate the third method embodimentof the invention, i.e., the addition of the adduct material to controlthe refractive index.

EXAMPLE 14

In this experiment 100 parts by volume of 11.4% sulfuric acid waspre-mixed with 3 parts of 1.38#/gal. aluminum sulfate. Thisacid-aluminum sulfate mixture was used to precipitate the aluminosilicate of controlled refractive index. Thirty-five (35) gallons of1.24#/gal. silicate of SiO₂ to Na₂ O molar ratio of 2.6 were added tothe stirred reactor and the silicate solution was heated to 175° F.Mixed acid-aluminum sulfate solution was added to the reactor at therate of 0.84 gallons per minute until reaction pH of 10.1 plus or minus0.1 was reached. At this pH, the precipitation of aluminum silicatemicelles just started. The acid-aluminum sulfate addition wasdiscontinued and the reaction medium was aged for fifteen minutes. Theaging or digestion step was found to be important to obtain ahomogeneous product and for the alumino silicate micelles to reach anequilibrium condition. After the aging period, both acid-aluminum andsilicate solutions were added simultaneously at the rate of 0.84 and 1.4gallons per minute, respectively. Silicate addition was turned off aftertwenty-five minutes; mixed acid-aluminum sulfate addition was continuedand the batch was finished off at pH 5.5, filtered, washed, dried, andmilled in the conventional manner. In a series of tests it was foundthat a range of products could be made by introducing more than oneaging step during the process and by maintaining the precipitation pHsubstantially constant and within the range of about 7-10.

EXAMPLE 15

A control experiment was performed by using the precipitation procedureof Example 14, but by using no aging step.

EXAMPLE 16

The procedure of Example 14 was repeated except the acid-aluminumsulfate ratio was varied by mixing 100 parts of 11.4% sulfuric acid with11.4 parts of 1.38 pounds per gallon aluminum sulfate.

EXAMPLE 17

A control experiment was performed as per the procedure of Example 16,but without using the aging step.

EXAMPLE 18

Example 14 was repeated by using acid-aluminum sulfate mixture of 100parts of acid to five parts of aluminum sulfate.

EXAMPLE 19

A control experiment was performed as per the procedure of Example 18but without using the aging step.

EXAMPLE 20

The procedure of Example 14 was repeated except that the acid andaluminum sulfate were pre-mixed in the ratio of 100:7.0.

EXAMPLE 21

The procedure of Example 14 was repeated but only acid was added up tothe aging step. After the aging period, acid, silicate, and aluminumsulfate were added simultaneously to the reactor. Both the silicate andaluminum sulfate additions were discontinued after twenty-five minutes.Acid addition was continued until final pH of 5.5 was obtained. Thebatch was processed similar to Example 13.

EXAMPLE 22

The procedure of Example 21 was repeated except that sodium silicatecontained 3% sodium sulfate. The effect of sodium sulfate was that thefinal product was more abrasive than the one produced in Example 20.

EXAMPLE 23

The procedure of Example 14 was repeated except that acid-aluminumsulfate were pre-mixed in the ratio 100 to 2.5.

The following Table (see Table III) summarizes the data obtained inExamples 14-23. From the following it will be seen that silicatepigments of controlled refractive index and abrasiveness can be tailoredby way of the process of this invention.

                                      TABLE III                                   __________________________________________________________________________                  %   %   (m.sup.2 /g)                                                                         Oil Abs.                                                                            Valley                                                                             Refractive                            Example                                                                            Description                                                                            Al.sub.2 O.sub.3                                                                  WCM Surface Area                                                                         (cc. 100g)                                                                          Abrasion                                                                           Index                                 __________________________________________________________________________    14   SAS* via aging                                                                         0.69                                                                              60  106    90    15.0 1.450                                 15   Control-no aging                                                                       0.69                                                                              79  161    203   2.7  1.446                                 16   SAS via aging                                                                          2.12                                                                              58  295    99    15.9 1.464                                 17   Control-no aging                                                                       2.12                                                                              76  250    203   3.6  1.464                                 18   SAS via aging                                                                          0.90                                                                              56  198    85    17.1 1.451                                 19   Control-no aging                                                                       0.90                                                                              79  232    219   2.3  1.451                                 20   SAS via aging                                                                          1.20                                                                              58  246    90    5.3  1.454                                 21   SAS via aging                                                                          0.70                                                                              58  202    99    9.0  1.448                                 22   SAS via aging                                                                          0.73                                                                              55  111    80    59.5 1.437                                 23   SAS via aging                                                                          --  56   87    74    11.5 1.450                                 __________________________________________________________________________     *SAS is Sodium Alumino Silicate                                          

From the above data, it will be seen that the new process results inalumino silicates of lower structure, lower wet cake moisture, lower oilabsorption and higher valley abrasion than the conventional precipitatedalumino silicates. Note that the controls of Examples 15, 17, and 19exhibit high oil absorption and low valley abrasions and hence cannot beused for clear gel toothpaste because they provide unacceptable abrasioncharacteristics and thicken up the toothpaste base even at low loadingsof up to only 8%. In general, refractive indices of from about 1.445 to1.475 are needed and required for clear gel cosmetic dentifrice use.

EXAMPLE 24

In a series of tests the general procedures of Examples 14-23 wererepeated except that nitric acid, hydrochloric acid, acetic acid, andformic acid were substituted for the sulfuric acid. The results weresubstantially the same as in Examples 14-23.

EXAMPLE 25

The general procedures of Examples 1-24 were repeated except thatpotassium silicate was substituted for the sodium silicate. The resultswere substantially the same as in Examples 1-24.

EXAMPLE 26

In this experiment the determination and variables associated with thefirst occurrance of the opalescence point as well as the importance ofthe "aging" at this point, were investigated. The study revealed that acritical step in producing an acceptable aged pigment is thedetermination of the opalescence point. For a given batch, theappearance of the apalescence point depends on the following:

Silicate concentration

Silicate mol ratio

Sulfuric acid concentration

Sulfuric acid rate

The development work on aged pigment was done by adding 11.4% sulfuricacid at the rate of 450 cc/min to 10 gallons of 2.5 mol ratio silicateof 13.3% concentration. The opalescence point appeared at 21 minutes and15 seconds of acid addition. When the same batch was prepared withsilicate of mol ratio 2.68, the opalescence point appeared at 19 minutesand 30 seconds. The processing advantages of the invention were found tobe directly related to the determination of the opalescence point. Ifthe reaction medium is aged by stopping acid about 30 seconds after theappearance of opalescence point, then about 50% of the processadvantages are lost. By aging the reaction medium 60 seconds after theopalescence point, 100% of the processing advantages are lost and theproduct has properties similar to those produced by prior knownprocesses. If the reaction medium is aged after stopping acid about 30seconds before the opalescence point, then an unacceptable product thatis slow filtering and which exhibits significant changes in propertiesis obtained. Thus, an acceptable product can be produced only bystopping the acid addition and aging the reaction medium as soon as theopalescence point appears.

In this regard a Bailey, high range bolometer, was hooked up to thepigment reactor to detect the opalescence point. It was observed thatsuch a sensing device can be used effectively in detecting theopalescence point. It is important that the bolometer chamber be free ofair bubbles; otherwise, a false opalescence point will be registered.

The following Table (Table IV) is a summary of the results of thisstudy.

                  TABLE IV                                                        ______________________________________                                                         Oil                                                                           Absorption M.R.  Final                                       % WCM  SA(m.sup.2 /g)                                                                          (cc oil/100g)                                                                            Silicate                                                                            pH    Remarks*                              ______________________________________                                        55     38        81         2.68  5.9   1                                     58     110       76         2.68  5.5   2                                     59     95        85         2.68  5.5   3                                     63     75        90         2.68  5.4   4                                     ______________________________________                                         *1=Aging done by stopping acid at opalescence                                 2=Aging done by stopping acid at 60 seconds before opalescence point          3=Aging done by stopping acid 30 seconds after opalescence point              4=Aging done by stopping acid 60 seconds after opalescence point         

From the above it will be seen that the process of the instant inventionresults in a new product having a unique combination of physical andchemical properties. These include, e.g., low abosrption, i.e., on theorder of less than 125 cc/100 gm, wet cake moisture contents of lessthan 70%, surface areas of less than 100 m² /g when the adduct materialis not added and in the range of from about 100-300 when the latter isemployed; pack densities of greater than 12#/ft³ and valley abrasions ofgreater than 5 (mg. wire loss). Improved and very important processingadvantages are also obtained. While particular embodiments have beendisclosed for illustrative purposes, the invention is not intended to belimited thereto. For example, in the case of pigment production for aspecial utility, the precipitating pH and the final slurry pH may betailored accordingly. Also, and as should be readily appreciated bythose skilled in the art, no special equipment is required in the methodherein described. In this regard, however, the reactor should beequipped with heating means, e.g., a steam jacket, in order to maintainthe desired reaction temperature and should have adequate agitatingmeans to produce a strong backflow on the body of the liquid and thusavoid zones of high concentration of the incoming reactants. It isdesirable to bring the reactants together so as to produce aninstantaneous reaction of all material being fed to the fullest extentreasonably possible, as such promotes uniformity of the resultingproducts. Storage vessels (for the reactants) connected to the reactionvessel thru lines fitted with flow control means may also be provided.The reaction vessel may be equipped with an outlet line leading to afilter which may be of conventional design. As noted above, the filteredmass is washed and dried. Such steps may also be conducted inconventional equipment it being understood, of course, that same do notform a part of the present invention.

If the pigments of the invention are used in toothpaste compositions,the dentifrice (if in the form of a paste) may contain humectantmaterials and binders to give the dentifrice a smooth texture and goodflowability. Glycerine, sorbitol, corn syrup glucose, and the like maybe used as carriers. Examples of binders include gum tragacanth, sodiumcarboxy-methylcellulose and the like. The above materials as well as thespecific formulation of the toothpaste are well known in the art and aredisclosed, for example, in U.S. Pat. No. 2,994,642 and 3,538,230, andnumerous publications.

As discussed above, the unique silicas of the invention may beadvantageously employed as abrasive or polishing agents in toothpastecompositions. This is truly remarkable inasmuch as precipitated silicasof the prior art cannot be so employed. If the products of the inventionare used in toothpaste compositions, and as known in the art, thedentifirce may contain, e.g., humectant minerals and binders to give thedentifrice a smooth texture and good flowability. The above materials,as well as the specific formulations of toothpastes, are well known inthe art and are disclosed for example in U.S. Pat. Nos. 2,994,642 and3,538,230 and numerous publications. A further detailed disclosure ofdentifrice formulations is given in U.S. Pat. No. 3,726,961.

In this regard, dentifrice formulations have been produced, ranging fromliquids and powders to the highly popular pastes or dental creams.Dental creams are the more difficult to compound successfully in thatthey require careful balancing of polishing agent, humectant, water,binder, preservatives, detergents, flavoring, sweeteners, andtherapeutic agents to produce a smooth homogeneous paste.

Most modern dental cream formulations use one of several phosphatematerials as the polishing agent. Examples of the phosphate polishingagents are dicalcium phosphate, anhydrous dicalcium phosphate,tricalcium phosphate, thermally converted dicalcium phosphate, andinsoluble sodium metaphosphate. The amount of phosphate materials addedto the dental formulations will range between about 5 percent and 60percent by weight.

The most widely used humectants in toothpaste are glycerine andsorbitol. Propylene glycol is also used in small amounts and to a verylimited extent. The primary function of humectant as part of the liquidphase is to retain moisture which provides good texture and maintains anattractive glossy appearance when the paste is exposed to air.

The binder employed therein is to prevent separation of the liquid andsolid phases. The most conventionally used binders are the seaweedcolloids and synthetic derivatives of cellulose, specificallyCarrageenan and sodium carboxymethyl cellulose. Others such as gums havebeen used. Combinations of these binders have also been employed.

Since the natural and synthetic water dispersions of organic binders aresubjected to microbial or mold attack, a relatively small amount ofpreservatives is added to the paste. Examples of preservatives used inthe industry are the esters of parahydroxyl benzoates.

The function of the detergents within the dental formulation is toprovide greater cleansing action due to the lowering of the surfacetension and the sudsing action in the mouth. Among detergents used aresodium N-lauryl sarcosinate, sodium lauryl sulfate, sulfoculaurate,sodium alkaly sulfoacetate, and sodium dioctyl sulfosuccinate.

Since toothpaste flavoring probably represents the greatest singlefactor in consumer acceptance, great care has been employed in selectingbalanced blends of different essential oils. These are rarely, if ever,used alone. Combinations of principal flavors are wintergreen,peppermint, and sassafras, and are used with secondary oils such aspimento, clove, and anise.

Saccharin and sodium cyclamate are widely used to improve taste andenhance the flavor qualities of the toothpaste. The synthetic sweetenersmay be used in combination to obtain optimum sweetness and absence ofafter-taste. Their desirable properties are obtained at very lowconcentrations and consequently they have negligible influence on thetoothpaste consistency.

Since water is such a common element, it is important in obtainingstable toothpaste formulations to employ substantially pure watertherein. It is common practice to demineralize the water that isemployed.

The therapeutic agents within the dental creams are to prevent decay ofthe tooth and are commonly in the form of stannous fluorides and sodiumfluoride material.

Difficulties have been encountered in using combinations of the abovematerials in modern dentifrice formulations. Specific scavenging of thefluoride ions by the phosphate and calcium containing polishing agentshave been experienced. Thus, in formulating a dentifrice composition, apolishing agent must be selected to provide excellent polishingproperties and have a very high degree of compatability with thefluoride system and in particular should not scavenge the fluoride ion.

The abrasivity index of new low structure silicas can be varied as afunction of structure index. The structure index of silicas can becontrolled by controlling the key processing parameters.

Thus one can tailor the new silicas to any RDA (Radioactive DentinAbrasion) values between 50 and 1200. The dentifrice formulators findtremendous flexibility in formulating a suitable dental care productwith these new silicas. This kind of flexibility does not exist whendentifrices are formulated from existing dentifrice grade phosphates.Furthermore, phosphate abrasives especially present fluoridecompatability problems when a therapeutic dentifrice is formulated. Thenew low structure silicas are known to enhance the cosmetic propertiesof the formulated paste when compared with a similar product formulatedfrom phosphate abrasive. FIG. 3 is a graph illustrating the abrasivityverses the structure index. As will be seen from this graph, the RDAdecreases as the structure index increases or, in other words, as thewet cake moisture increases.

What is claimed is:
 1. A visually clear toothpaste compositioncomprising a particulate solid polishing agent in a liquid phase,characterized in that the polishing agent comprises a syntheticprecipitated amorphous silica, said synthetic precipitated amorphoussilica being present in said composition in an amount of from about 15to about 50% by weight of the toothpaste, and said syntheticprecipitated silica having substantially the same refractive index assaid liquid phase; and said amorphous silica comprising silica micellesor aggregates which have been milled to the desired degree of fineness;said silica being effective to function to remove stains, food, debris,and plaque from the human tooth surface by providing good cleaningaction at acceptable abrasion levels without imparting undesirableopacity to the toothpaste and having a Radioactive Dentin Abrasion valuein the range of at least above 50, said silica aggregates thereby beingsubstantially free of precipitated silicas which have substantially nocleaning ability on human teeth, and have initially small particlesizes, and are substantially free of the ease of breakdown into smallparticle sizes, thus resulting in good cleaning ability.
 2. A visuallyclear toothpaste composition comprising a particulate solid polishingagent in a liquid phase, characterized in that the polishing agentcomprises a synthetic precipitated amorphous silica, wherein saidprecipitated amorphous silica contains a sufficient amount of an adductmaterial selected from the group consisting of aluminum, magnesium,zinc, and calcium, to raise the refractive index of said precipitatedsilica to the range of above 1.475, said synthetic precipitatedamorphous silica being present in said composition in an amount of fromabout 15 to about 50% by weight of the toothpaste, and said syntheticprecipitated silica having substantially the same refractive index assaid liquid phase; and said amorphous silica comprising silica micellesor aggregates which have been milled to the desired degree of fineness;said silica being effective to function to remove stains, food, debris,and plaque from the human tooth surface by providing good cleaningaction at acceptable abrasion levels without imparting undesirableopacity to the toothpaste and having a Radioactive Dentin Abrasion valuein the range of at least above 50, said silica aggregates thereby beingsubstantially free of precipitated silicas which have substantially nocleaning ability on human teeth and have initially small particle sizes,and are substantially free of the ease of breakdown into small particlesizes, thus resulting in good cleaning ability.